function checkForm() {

    //判断用户输入的登录名与密码
    var userName = document.getElementById("username").value;
    var passWord = document.getElementById("password").value;
    if (userName == null || userName == "") {
        alert("账号不能为空");
        return false;
    }
    if (passWord == null || passWord == "") {
        alert("密码不能为空");
        return false;
    }
    return true;

}